Geology Reference
In-Depth Information
(a)
(b)
D
3
Surface
D
1
D
2
Datum
S
1
D
1
S
2
D
2
S
3
D
3
Datum
V
w
V
1
B
S
3
S
1
V
1
S
2
t
12
t
12
t
12
S
1
D
1
S
2
D
2
S
3
D
3
S
1
D
1
S
2
D
2
S
3
D
3
t
t
23
t
23
t
23
Fig. 4.15
Static corrections. (a) Seismograms showing time differences between reflection events on adjacent seismograms due to the
different elevations of shots and detectors and the presence of a weathered layer. (b) The same seismograms after the application of elevation
and weathering corrections, showing good alignment of the reflection events. (After O'Brien, 1974.)
to the shot hole to measure the
vertical time
(
VT
) or
uphole
time
, from which the velocity of the surface layer above
the shot may be calculated.
The complex variations in velocity and thickness
within the weathered layer can never be precisely de-
fined. The best estimate of the static correction derived
from the field data is usually referred to as the
field static
.
It always contains errors, or residuals, which have the
effect of diminishing the SNR of CMP stacks and
reducing the coherence of reflection events on time
sections. These residuals can be investigated using so-
phisticated statistical analysis in a
residual static analysis
.
This purely empirical approach assumes that the weath-
ered layer and surface relief are the only cause of irregu-
larities in the travel times of rays reflected from a shallow
interface. It then operates by searching through all the
data traces for systematic residual effects associated with
individual shot and detector locations and applying these
as corrections to the individual traces before the CMP
stack. Figure 4.16 shows the marked improvement in
SNR and reflection coherence achievable by the appli-
cation of these automatically computed residual static
corrections.
In marine reflection surveys the situation is much
simpler since the shot and receivers are situated in a
medium with a level surface and a constant velocity.
The static correction is commonly restricted to a con-
version of travel times to mean sea-level datum, without
removing the overall effect of the water layer. Travel
times are increased by (
d
s
+
d
h
)
v
w
, where
d
s
and
d
h
are
the depths below mean sea-level of the source and
hydrophone array and
v
w
is the seismic velocity of sea
water. The effect of marine tidal height is often signifi-
cant, especially in coastal waters, and demands a time-
variant static correction. Tidal height data are usually
readily available and the only complexity to the correc-
tion is their time-variant nature.
4.7 Velocity analysis
The
dynamic correction
is applied to reflection times to re-
move the effect of normal moveout. The correction is
therefore numerically equal to the NMO and, as such, is
a function of offset, velocity and reflector depth. Conse-
quently, the correction has to be calculated separately for
each time increment of a seismic trace.
Adequate correction for normal moveout is depen-
dent on the use of accurate velocities. In common mid-
point surveys the appropriate velocity is derived by
